The present invention relates to an electronic endoscope system to be used for displaying a color image of an object.
Generally, an electronic endoscope system is provided with an endoscope having a flexible tube accommodating an image sensor (such as a CCD) (Charge Coupled Device) and an objective lens at a distal end thereof. An image of an object is formed on an image receiving surface of the CCD by the objective lens system. The flexible tube also accommodates an optical fiber bundle. The distal end of the optical fiber bundle is located at the distal end of the flexible tube, and the proximal (operator-side) end of the optical fiber bundle is coupled to a light source unit to face to a light source included in the light source unit. Accordingly, light emitted by the light source is transmitted through the optical fiber bundle and illuminates the object.
The CCD includes a plurality of pixels. However, since the number of pixels for the CCD used in the endoscope is relatively small in comparison to CCDs used for camcorders or the like, in order to obtain a color image signal of the object, a color sequential method is used. Specifically, a rotary RGB color filter is provided between the optical fiber bundle and the light source inside the light source unit. By inserting each of an R (Red), G (Green) and B (Blue) filter sequentially into the optical path from the light source to the fiber bundle, red, green and blue light is sequentially transmitted through the fiber bundle to illuminate the object. Synchronously with a charge of the color illuminating the object, the CCD is driven to output an image signal. Accordingly, a red image signal (referred to as an R signal hereinafter), a green image signal (referred to as a G signal hereinafter), and a blue image signal (referred to as a B signal hereinafter) for one frame of an image to be displayed are sequentially output by the CCD. The R, G and B signals are processed, a video signal is created, and a color display is driven to display a color image. This process for obtaining the color image signals for one frame (or one field) sequentially, and for generating the color image based on each color image signal, is well known as a field sequential type color imaging process.
The R, G and B signals obtained as described above (i.e., in accordance with the field sequential type color imaging process) generally include random noise. For example, since the object is generally inside a human cavity, the object is illuminated only by the light emitted from the end surface of the fiber bundle, and thus, the amount of light which the CCD receives may vary greatly depending on the positioning of the distal end of the endoscope. Such a variation of the light amount may appear as noise and affect the displayed image. Further, other noise such as that due to dark current or the like may also affect the displayed image. If an observation is completed within a relatively short period noise may not be a large problem, however, if the observation continues for a relatively long period, noise may cause strain on the observer's eyes and an accurate diagnosis may not be made.